Hydraulic cement, particularly Portland cement, is commonly used throughout construction in many applications. Recent developments in polymer latex technology have provided compatible polymer emulsions and compounds for use in conjunction with these hydraulic cements alone for improvement of properties such as bond strength, tensile strength, and flexural strength.
There have been numerous prior attempts at providing cementitious systems to meet the needs of the construction industry, particularly in the protection, waterproofing, and repair of concrete structures. The optimum system should set within a relatively short period of time into a hard mass or coating that has sufficient strength, abrasion resistance, and corrosion resistance. It is also highly desirable that these systems possess impermeability to fluids, particularly aqueous solutions. Also, such systems should not undergo excessive hardened volume changes under either wet or dry conditions.
For commercial use, these types of cementitious systems must also possess good bonding characteristics to damp or dry surfaces, early as well as long term strength, and practical field workability. They should be capable of withstanding freezing and thawing, as well as the action of salts, solvents and other corrosive substances. Although there have been a number of cementitious mixtures that possess one or more of the above-described desirable properties, none of the prior art to date has been able to achieve all of the foregoing in one composition and previous attempts have only had limited success. U.S. Pat. No. 4,357,166 discusses some of the limitations of these prior art compositions in columns 2 and 3, and is expressly incorporated herein by reference.
It has been well known that cementitious mixtures in general and Portland/gypsum mixtures in particular demonstrate poor durability, weatherability, freeze/thaw resistance, poor resistance to water and salt penetration, erosion, and long term dimensional stability when wet, as well as exhibit poor bonding performance. In addition, varying shrinkage and expansive results can occur due to variations of tricalcium aluminate and other compounds in the Portland cement. Moreover, previous attempts to control volume change as well as to produce a durable, permanent, high bonding, moisture resistant, freeze/thaw resistant, crack-free, and sound cementitious mixture have resulted in limited or no success. In addition, these Portland/gypsum compositions demonstrate higher porosity and absorption rates than equivalent Portland cement mixtures alone, thereby jeopardizing not only the resistance of the material to corrosive water or salts, but also to freeze/thaw cycling. The Portland/gypsum mixture's , performance as patching, topping, and resurfacing materials to protect concrete from the action of salts has also been poor. It is well known that gypsum materials alone or in combination with Portland cements do not have the water resistance nor the resistance to weathering or long term durability under wet/dry cycling.
In addition, it has been noted that production Portland cements vary in both fineness and composition so that Portland/gypsum mixtures vary significantly in both set time, water demand, and consistency, thus resulting in variable field performance and mixing requirements as well as unpredictable early strengths. Constant plant adjustments and variations in formulations and appropriate admixtures to be undertaken to provide the product with reasonably reliable performance.
Previous compositions containing other hydraulic cements such as aluminous cement and gypsum have been shown to exhibit long term wet expansion. For example, U.S. Pat. No. 4,357,166 discloses a cementitious composition which when mixed with water is capable of setting rapidly to a hard mass of high compressive strength without substantial shrinkage during setting and which exhibits reduced wet and dry volume changes in the hardened state. That invention also possesses a degree of impermeability to fluids along with abrasion, erosion, and chemical resistance, as well as other characteristics which are desirable in a composition having commercial usefulness in the construction industry. The advantages of that invention are achieved by a cementitious composition comprising a mixture of an aluminous cement, a gypsum, a drying shrinkage inhibitor, and a wet expansion inhibitor. The compositions of that invention preferably use Portland cement as a drying shrinkage inhibitor and a lithium salt as the wet expansion inhibitor. However, various accelerators, retarders and other admixtures, when added to aluminous cement and gypsum compositions, can significantly affect the hardened volume change (wet or dry), thus limiting their usefulness.
Additionally, previous attempts to blend gypsum with other hydraulic cements, while producing the desired effect such as fast-setting or reduced shrinkage, have also affected the soundness, durability, workability, resistance to water or wet/dry cycling stability or permanency of the resulting cement.
The addition of latex emulsions or compounds to Portland cements alone has been well known and used to improve bond strength, tensile strength, absorption rates, and the like. In addition, latex emulsion compounds more recently have disclosed for use in expansive cements utilizing mechanical restraining devices such as reinforcement of restraining elements to control or reduce volume changes. However, the stresses created by these may still cause cracking, curling or debonding from weak surfaces such as foam glass. In addition, they require careful design, mapping, spacing or dispersion to provide uniformly effective reinforcement. (See, e.g., U.S. Pat. No. 4,039,345). Also, formulations of expansive cements that have utilized latexes produce an initial wet or dry expansion without eliminating the problem of eventual drying shrinkage of similar magnitude of an equivalent ordinary cement drying out unrestrained with or without the use of latex emulsions and compounds.
It is also known that polymer latex emulsions can be utilized with Portland cement mortars to increase their tensile strengths and resistance to water penetration. One would expect, however, that the addition of a polymer latex emulsion to a HAC/gypsum composition would hinder or inhibit wet expansion because, theoretically, the latex should prevent water from entering the mix, thus limiting the amount of additional water needed to cause wet expansion through the formation of ettringite. In addition, by increasing the tensile strength of the hardened HAC/gypsum composition, one would expect that the polymer latex compound would reduce any tendency of the composition to expand by creating an internal restrained effect.
In addition, latex has been added to gypsum mixtures for use as drywall joint compounds, as shown in U.S. Pat. No. 4,294,622. Also, U.S. Pat. No. 4,088,804 shows that latex has been added to Portland cement or high alumina cement as a protective or decorative coating, about 2 mils or greater thickness with finely divided mineral aggregates, water reducers and rust inhibitors. High alumina cement, lithium, and latex combinations have also been noted in U.S. Pat. No. 4,352,693.
Traditionally, cement-based stucco materials and protective coatings for rigid insulation systems have required the use of metal lath, fiber, and mesh to reduce or control cracking due to shrinkage in cementitious mixtures.
It is well known that the addition of latex to Portland cement and other sand hydraulic cements improves bonding performance and increases the tensile and flexural strength. However, the addition of latexes to such cements alone does not specifically reduce ultimate drying shrinkage, and in certain instances increases drying shrinkage except when latex solids are merely used to reduce the water/cement ratio by substituting polymer solids for water. Furthermore, certain latex emulsions in combination with one or more cements can cause unsoundness, excessive expansion, nonuniform surface finishes, retarded set time and/or poor workability and finishing properties.
Previous applications of cementitious compositions to porous, absorptive concrete and masonry type surfaces have required surface wet down or substrate saturation prior to the application of the cementitious coating in order to retard or eliminate the tendency of the absorptive substrate from absorbing water from the freshly applied cementitious coating. The rapid loss of mixing water from the freshly placed cementitious coating due to this absorptive nature of the substrate leads to a number of undesirable and detrimental effects, such as rapid loss of workability, plastic shrinkage cracking, lower strengths, poorer bonding, softness of the cured coating, and many other properties of hydraulic cements which are dependent upon the presence of curing water for proper development, i.e., permeability and the like.
In addition, the application of cementitious coatings and numerous other concrete type products, such as patching, topping, resurfacing, and the like, require the application of a curing compound or moist curing process which may involve numerous repeated steps of fogging or the application of and protection with wet burlap and the like in order to reduce plastic shrinkage cracking and the rate of drying shrinkage as well as providing proper amounts of moisture in order for the hydraulic cement to properly hydrate and increase in strength and other such related properties.
Under a number of field conditions, such as hot or windy weather, it can become very costly, if not impossible, to keep absorptive surfaces dampened or saturated in order to obtain proper material performance in the field. In addition, any deep in-ground structures, as well as high rise structural faces which are not easily accessible, cannot easily or practically protected from premature drying due to excessive evaporation after application.
These problems become particuarly evident when workers are highly dependent upon scaffolding and other such remote work platforms. Furthermore, reliability of material that requires thorough saturation of the substrate and moist curing after application becomes highly dependent upon the timing, thoroughness and experience of workmen.